Commercially sterile-treated food products, for example milk products or juice, are nowadays often packed in consumer packages which are manufactured from previously sterilized or sterile-treated packaging material. One typical such package is the parallelepipedic packaging container principally for liquid contents which is known under the name Tetra Brik Aseptic.RTM. which is manufactured by folding and heat sealing of web-shaped packaging laminate. The packaging laminate comprises layers of fibre material, for example paper, which are coated on either side with thermoplastic material, normally polyethylene. On that side of the packaging laminate which is to be turned to face towards the contents of the container, there is also a layer of a suitable barrier material, normally aluminium foil, which in turn is coated with a thermoplastic layer.
In the manufacture of parallelepipedic packaging containers of the above-mentioned type, use is made of packaging or filling machines which are fed with the packaging material in web form. The packaging material web is sterilized on its passage through the packaging machine in that its surface is coated with a chemical sterilization agent, e.g. hydrogen peroxide, which is subsequently once again removed mechanically from the surface or is vaporised by heat. Hereafter, the packaging material web is reformed while being located in a closed, sterile space, into tube form whereafter the tube is filled with previously sterilized contents and is transversely sealed and severed into cushion-shaped, filled and sealed packaging containers. These are then reformed by mechanical processing into substantially parallelepipedic form. The contents have normally been sterilized beforehand by heat treatment, so-called UHT treatment (ultra high temperature). Because of the relative absence of bacteria, packaging containers produced by such means enjoy an extremely long shelf life even when stored at room temperature. Although the manufacturing method is extremely well-tested and reliable, it cannot, of course, be guaranteed with absolute certainty that all the packages in any batch of any size be sterile. It is therefore normal to undertake, during production, certain controls with a view to establishing the quality of the packed product from the aseptic point of view. Normally, this known control is effected in that a number of newly produced packaging containers are stored for a predetermined time at suitable temperature (e.g. 7 days, 30.degree. C.). After the predetermined storage time, the packaging containers are opened and inspection is carried out of the contents of the package by measuring the pH, by plating on culture medium or by other suitable method for detecting the micro biological growth in the product. Since micro biological growth changes the electrical properties of the substrate, this test can also be put into effect in practice in that, after the packaging containers have been stored at a suitable temperature for a predetermined amount of time (e.g. 2 days, 30.degree. C.), a certain quantity of product from each opened packaging container is poured into a special measurement cell, either containing culture medium or not, where micro biological growth can be detected from the change in electrical properties (conductance and/or capacitance and/or impedance).
In connection with the above-mentioned inspection and control of the quality of the contents (or separately), inspection and control are often carried out of the tightness of the packaging container wall or the material. In order, as far as is possible, also to cover damage which may have occurred during the forming processing work, such inspection and control are normally carried out in the form of a destructive control in connection with the above-mentioned control of the quality of the contents. This control principally focuses on the inner thermoplastic layer of the packaging laminate, since any possible leakage in this layer may entail that the contents reach the barrier layer (the aluminium foil) or the fibre layer, in which event the other barrier properties of the laminate are lost, even if no actual liquid leakage occurs through the laminate. This type of liquid tightness control has hitherto been difficult and time-consuming to carry out with reasonable demands on reliability and costs.
As will have been apparent from the foregoing discussion, the known testing and control method principally suffers from the drawback that all tested packaging containers must be opened and so destroyed. Thus, previously incubated packaging containers which do not display any unacceptable micro biological growth (or leakage) are also lost, which naturally occasions unnecessary costs and work. Hence, it is desirable in the Art to be able to provide a method for quality control of contents or packaging containers of the above-outlined type which considerably reduces loss and costs in connection with quality control of produced packaging containers.